Light engines (often referred to as “illuminators”) are used in many applications, and especially in fiber optic illumination applications. Light engines are typically used to locate a source of light remotely from an area to be illuminated. Several advantages are provided by being able to locate the light engine remotely from the area where the light it produces is to be used. Safety, spectrum control, thermal concerns, etc., are but a few reasons why it is often desirable to locate the light engine remotely from an area or object that is to be illuminated.
The problem with using light engines in fiber optic illumination systems is the needed efficiency. Direct illumination by a light source is the more efficient means for providing light. For fiber fed illuminators to compete with direct illumination systems, the efficiency and the amount of light transported by a fiber (or optical fiber bundle) needs to approach the system efficiency of a direct illumination system.
A specific limitation with present day fiber fed illuminators is the criticality of the diameter of the spot size of the beam of light produced by the illuminator. Ideally, the spot size should be minimized so as to better concentrate and focus the optical energy of the beam into the input face of an optical fiber or optical fiber bundle. There have been many attempts, with limited success, to control the spot size in an effort to reduce it so that the light beam from an illuminator can be focused into a smaller diameter optical fiber or a smaller diameter optical fiber bundle.
Many present day fiber fed illuminators incorporate some form of reflector system which is used to more closely focus the light from a light source of the illuminator. Such a system is shown in FIG. 1. The spot size of a beam produced by any given reflector system is governed by the physical size of the light source, the solid angle of the source radiation pattern and the numerical aperture (NA) of the wave guide, light pipe or fiber used to receive the reflected light. In FIG. 1, it will be noted that the light source “L” disposed at a focus (i.e., focal plane) of a reflector “R” produces a large beam (“spot”) which covers substantially an entire face of a target “T”. This is partly because “low angle” light rays, represented by dashed lines “D” are reflected by the reflector R, which produce a very large spot at the target. It would be much more desirable if only the more accurate “high angle” rays, such as rays “H”, were reflected at the target T. However, forming the reflector such that the high angle rays are reflected results in a loss of a significant portion of the optical energy from the light source L. The use of various forms of reflectors alone, and in combination with a condensing lens, has failed to achieve a significant reduction in the spot size of the reflected beam. FIG. 2 illustrates a prior art dual ellipsoid reflector and the equations for predicting the distribution of the spot size of the beam reflected onto a target plane.
Many present day approaches which attempt to reduce the spot size of the light beam from a fiber fed illuminator make use of either a standard ellipsoidal reflector, a dual ellipsoidal reflector, a parabaloid reflector with some form of optical lens, and various other facetized versions of these approaches. All of these approaches are subject to a common geometric limitation. That limitation is that while a typical ellipsoidal reflector may very accurately direct the light source to an output location (i.e., focus) at source points close to the median of the ellipse, the reflected light diverges away from the output location as the source point moves away from the median of the ellipse. The spot size is governed by the numerical aperture (NA) of the accepting target, the solid angle of the source radiation pattern and the source's physical size.
In view of the above, it will be appreciated then that a standard ellipsoidal reflector has a geometric limitation for the spot size that it can produce. There have been many attempts to “piece wise” control the distribution of the output by facetizing the reflector. Facetized reflectors are designed to “tweak” the distribution of light at the target by orienting areas on the reflector surface (facets) in order to meet some predetermined output beam pattern. However, facetized reflectors still may not actually focus the source light better, but can sometimes distribute the light to better meet some predetermined requirement. More precise control of the output of the light source would allow even more light to be focused into a smaller diameter spot. In practical terms, this would allow for a smaller diameter optical fiber or optical fiber bundle to be used to receive the optical signal from the signal source to handle a given illumination task.
Accordingly, there still exists a need for a fiber fed light engine which is able to more closely focus a light beam from a light source in a manner that reduces the spot size of the beam to a greater degree than what is possible with present day light engines. Reducing the spot size of the beam would allow smaller diameter optical fibers and optical fiber bundles to be employed, which would significantly improve the overall efficiently of the system, in addition to reducing the overall cost and weight of a fiber optic illumination system.